1. Field of the Invention
The present invention relates to a two-dimensional photonic crystal slab included in a micro-optical circuit element or the like, a two-dimensional photonic crystal waveguide including such a two-dimensional photonic crystal slab having a linear defect, and an optical device including such a two-dimensional photonic crystal waveguide.
2. Description of the Related Art
Materials with a periodic variation in refractive index on a length scale comparable to the wavelength of light are referred to as photonic crystals, which have forbidden bands that is, so-called photonic band gaps. In such materials, the propagation of light with a specific wavelength corresponding to the periodicity is prohibited. In recent years, the photonic crystals have been attracting much attention because light propagation can be precisely controlled using the photonic crystals, which therefore seem to be suitable for next-generation electronics or optoelectronics.
FIG. 44 shows an example of known two-dimensional photonic crystal waveguides (see Japanese Unexamined Patent Application Publication No. 2001-272555).
This two-dimensional photonic crystal waveguide includes a slab 81, made of a material having a refractive index higher than that of air, having cylindrical holes 86 arranged in a triangular grid pattern. The slab 81 therefore functions as a two-dimensional photonic crystal. The slab 81 has a linear defect section 92 extending between the cylindrical holes 86 as shown in FIG. 44. The linear defect section 92 functions as a waveguide section.
When the two-dimensional photonic crystal waveguide is irradiated with a light beam 103 with a wavelength corresponding to a frequency within a photonic band gap, the light beam 103 is prevented from being propagated in regions of the waveguide other than the linear defect section 92 because the regions have photonic band gaps in the in-plane direction; however, the light beam 103 is propagated in the linear detect section 92 functioning as a waveguide section and confined in the direction perpendicular to the in-plane direction by the total internal reflection due to a difference in refractive index.
Known two-dimensional photonic crystal waveguides have a photonic band gap for only one of a light beam in the transverse-electric (TE)-like mode and a light beam in the transverse-magnetic (TM)-like mode, the TE-like mode and the TM-like mode being among polarization modes. Therefore, a light beam in one of the modes is prevented from being propagated but a light beam in the other leaks in the in-plane direction of the waveguides; hence, the waveguides have low light extraction efficiency. For example, a two-dimensional photonic crystal waveguide having cylindrical holes arranged in a triangular grid pattern has a photonic band gap only for the light beam in the TE-like mode; hence, the light beam in the TM-like mode leaks in the in-plane direction of the waveguide.
A two-dimensional photonic crystal slab having a photonic band gap common to the light beam in the TE-like mode and the light beam in the TM-like mode has been demanded; however, such a two-dimensional photonic crystal slab has not been obtained.